Present disk memory devices use magnetic or optical methods to write (or read) digital data onto a magnetic or optical disk. For either data recording method, the present art allocates the memory surface of the disk in two basic ways. One way is to write (or read) the data onto a single, narrow spiral track that runs from the edge of the disk towards the center of the disk. An example of this is the Compact Disk. Another technique allocates the surface into many, narrow concentric tracks. This is the usual approach to a computer disk memory. For this present invention description, the many concentric data tracks will be assumed, but there is an equivalent implementation of the invention for the single spiral data track allocation method.
For all disks, the innermost track is always somewhat shorter than the outermost or edge track. For present art, the outer-track to inner-track length is typically 2:1. If the bit packing density along the track were constant, the edge track would be able to store twice the data that can be stored on the innermost track of the disk. Unfortunately, present art either wastes this storage capacity or, if this capacity is used, constrains the data rate and/or data access time. Three examples of the present art are discussed below.
Many present art disk memory devices use a fixed data rate and a fixed disk rotational speed. An example of this type of device is the disk memories used in computers. The number of bits per track on the disk is then fixed at whatever can be stored along the shortest, innermost track and the maximum data rate is set by the time it takes the disk to make one revolution. For a 2:1 outertrack to inner-track ratio, this present art does not utilize 25% of the available storage area.
Another present art disk memory device uses the full capacity of the disk but varies the disk rotational speed depending on which part of the disk is being used at the time. In this case, the maximum data rate is set by the maximum rotational speed and the number of bits on the last track. This fully uses the memory area and, with a sacrifice in data rate, allows going to larger outertrack to inner-track length ratios. For instance, a 3:1 ratio gives an 18.5% increase in data capacity for the disk but with a data rate reduction of 33.3%. This art is adequate for certain applications such as when the data are played sequentially from one end to the other, for example, such as in a video recording disk. However, this technique is not practical as a computer storage device. Computer applications require random accessing one track or another and the time for the disk to achieve the correct speed when jumping among the various data tracks would be unacceptably long.
A third possibility for implementing a disk memory device with the present art would be to use a constant rotational rate and allow the data rate to vary depending on which track was being used For a 3:1 ratio of track lengths, a track at the disk's edge would produce a data rate three times higher than the innermost track would produce. This would maximize the number of bits that could be stored on the disk, but the variable data rate would not be acceptable for most user of the memory.